A variety of capacitor discharge circuits has been employed in prior-art systems. Discharge circuits are used to remove an electrical charge from a capacitor when power to the capacitor is turned off. Conventional, prior-art circuits utilize a "bleed" resistor connected in parallel with the capacitor to be discharged. While this simple circuit effectively discharges the capacitor, the time duration for discharge can be undesirably long in applications requiring large capacitors. Furthermore, certain circuit applications, such as circuits employed for detonating a pyrotechnic device, require substantially instantaneous discharge times, which are not practicably obtainable with "bleed" resistors, for preventing accidental triggering of the explosive load by voltage accumulation in the circuit's capacitive elements. The low value of resistance required for a bleed resistor to accomplish rapid capacitor discharge results in low operating efficiency when power is being applied to the capacitor, and may not provide sufficient protection against undesired voltage accumulation.
One solution to this problem is to utilize the avalanche characteristics of a Zener diode. This approach is used in U.S. Pat. No. 3,100,269, issued Aug. 6, 1963 to D. T. Barry, which discloses a quick acting gate for voltage or current discharge. The avalanche effect is achieved by applying a reverse voltage equivalent to the Zener diode's breakdown voltage, resulting in its reverse bias of the diode, and thus providing a low impedance current path through which the capacitor is discharged. However, such a circuit requires a means for biasing the Zener diode to maintain a non-conducting state during normal circuit operation, as well as circuit means for providing a trigger pulse, i.e., the voltage necessary to cause breakdown of the Zener diode. This double source requirement adds to the cost and complexity of the circuit.
Other proposed means for rapid discharge of a capacitive load include the use of a transformer which, upon some determinative change in the primary circuit's current flow, causes current to flow through the secondary circuit, thus triggering a semiconductor which acts as a current path through which the capacitive load is discharged. Again, such circuits are undesirably complex and expensive. There is, thus, a need for an efficient, inexpensive rapid discharge circuit having no peripheral triggering power source or device added to the primary circuit.
An object of the invention is to provide a protective circuit for preventing undesired accumulation of voltage in capacitive elements of time delayed detonation circuits.
A further object of the invention is to effectively isolate a detonation circuit from a commonly shared, multiple purpose power source to prevent intermittent pulse signals generated by the power source from inducing an accumulation of charge in capacitive elements of the detonation circuit.